1. Field of the Invention
The field of the invention is AC motor drives for variable speed control of AC induction motors, and more particularly, AC motor drives using pulse width modulation (PWM) techniques.
2. Description of the Background Art
A PWM motor drive for an AC induction motor includes a power section and a logic and control section. The power section includes the high voltage and current devices to convert AC input power to DC power and then, to convert the DC power to a variable frequency, PWM voltage for input to an AC motor. The logic and control section includes the low power signal processing circuits and logic circuits which control the performance characteristics of the drive. The user of the drive sets certain operating parameters through input devices interfaced to the logic and control section.
In the power section a PWM voltage inverter receives power from a 3-phase AC source operating at 60 Hz frequency. The AC power is converted to DC power to provide a source for synthesizing voltages of different frequencies which are necessary to control speed in an AC motor.
The pulse train pattern from a PWM inverter is characterized by a first set of positive pulses of equal magnitude but of varying pulse width and by a second set of negative-going pulses of equal magnitude but of varying pulse width. The RMS value of this pulse train pattern approximates one cycle of a sinusoidal signal which is characteristic of an AC waveform. The pattern is repeated to generate additional cycles of the AC waveform.
To control the frequency and magnitude of the resultant AC power signals to the motor, control signals from the logic and control section are applied to the PWM inverter.
AC motor control systems that incorporate PWM drives, can be categorized as follows: 1) a closed-loop type in which the speed of the motor is sensed with a tachometer and fed back to determine an error signal which is applied to reduce the difference between a commanded speed and the actual speed, and 2) an open-loop type which does not include a tachometer for sensing the actual speed of the motor.
The advantage of the open-loop type is lower cost, in that the tachometer is a relatively sophisticated and expensive accessory to the basic motor control. The tachometer is also sometimes difficult to connect and may cause a decrease in overall system reliability. Controls without speed sensing are considered to be "open-loop", even though other parameters such as voltage or current may be sensed.
To drive a PWM voltage inverter the motor control provides an AC signal of a certain magnitude and frequency. In one type of open-loop control, slip is roughly determined and frequency is more closely determined by predefined speed profiles, referred to as "accel/decel" rates which are selected and adjusted through switches interfaced to the logic and control section of the motor control. The speed commands are translated into torque commands by applying a specified volts/hertz ratio, which can be selected through a user-selectable switch or a jumper wire interfaced to the logic and control section of the motor control.
A PWM voltage inverter provides fast transient response to load disturbances which is a significant advantage. A second advantage is that velocity feedback is not necessary to reach a stable steady-state operating condition. However, there are times, when starting the motor or when strong load disturbances occur, when current to the motor may become excessive.
One way of controlling current at startup is to provide a current regulator which compares a commanded current to the actual current and generates an error signal to the PWM inverter to reduce the error. A current-regulated PWM (CRPWM) drive, however, does not respond as quickly as a voltage inverter to sudden and substantial load disturbances.
For overcurrent protection in a voltage inverter drive, a circuit is available for monitoring current to the motor and according to the specific condition either shutting down the motor or providing a controlled slow-down and re-acceleration to operating speed. This solution has not entirely eliminated undesirable torque oscillations during certain extreme loading conditions. Also, during light loading, it has been found that the motor may be overexcited resulting in inefficient performance. Still further, the shutting down of the motor in response to load disturbances is in some instances an unnecessary inconvenience to the motor user.
Thus, it is an object of the present invention to improve the response and control capabilities of an open-loop PWM motor control without increasing its production cost by an amount comparable to the cost of adding speed sensing equipment.